The best example, where the needed rotational speed or torque is variable in relation to the supplied rotational speed or torque, is that of the motor vehicle. To achieve the above, motor vehicles usually employ a manual, an automatic or a type of continuously variable transmission, from now on referred to as ‘CVT’. Examples of continuously variable transmissions include devices using a belt with conical pulley systems, friction disc transmissions, hydraulic traction fluid based systems and ratcheting CVTs.
In manual transmissions, gear-ratio is varied by engaging different gear pairs of various diameters. In automatic transmissions the gear-ratio is varied usually by employing planetary gears and by altering, which gears act as the input gears and which act as the output gears. In manual transmissions a clutch or similar mechanism is usually used to disengage the source of torque from the input and output gears during gear ratio change. In automatic transmissions, friction clutches, brake bands and, friction brakes are used to halt one line of force transfer and release another.
In continuously variable transmissions using a belt with conical pulley systems, and in friction disc transmissions, gear-ratio is varied by altering the torque transferring site of contact between frictional surfaces. Thus, no clutches or brake bands or friction brakes are needed for gear-change. However, torque is transferred through frictional surfaces. In hydraulic CVTs all power is transmitted by hydraulic fluid. Gear-ratio change is achieved by the use of a variable displacement pump which then sends hydraulic fluid at different rates to a hydraulic motor. In ratcheting CVT a set of elements, successively become engaged and then disengaged between the driving system and the driven system, usually using oscillating or indexing motion in conjunction with one-way clutches or ratchets that rectify and sum only one way motion. The gear-ratio is adjusted by changing linkage geometry within the oscillating elements, so that the summed maximum linkage speed is adjusted, even when the average linkage speed remains constant.
Clutches or torque converters are used in association with most transmissions of the prior art, to enable the engine to run while the wheels are stationary and also to bring about vehicle launch. An exception is the infinitely variable transmission, referred to as ‘IVT’ which results from the combination of a CVT with an epicyclic gear system.